Environmental Engineering Reference
In-Depth Information
Keywords biomolecules
catalysts
clays
early Earth atmosphere
homochirality
Please cite as:
Met. Ions Life Sci.
14 (2014) 1-14
1 The Early Earth Atmosphere and Lithosphere
The solar system condensed out of an interstellar cloud of gas and dust, known as
the primordial solar nebula, about 4.6
10
9
years ago. The formation of the inner
planets was the result of innumerable impacts of small, rocky particles in the solar
system's protoplanetary disk. Over a period of a few million years, neighboring
dust grains and pebbles in the solar nebula coalesced into larger objects called
planetesimals. Their mutual gravitational attraction brought a few of them together
to form larger objects, known as protoplanets. The accretion of more material
continued for another 100 million years and led to the formation of the terrestrial
planets: Mercury, Venus, Earth, and Mars [
1
].
The theory of planet formation and the models that attempt to reproduce the
early history of Earth indicate that the actual atmosphere is the third atmosphere to
evolve [
1
]. The earliest Earth's atmosphere was composed of trace remnants of
dihydrogen (H
2
) and helium (He) left over from the formation of the solar system,
but it did not last very long as these gases are too light to be kept. They gained enough
energy from the sunlight to overcome the Earth's gravitational attraction and moved
away into space [
1
]. The gases of the second atmosphere came from inside the Earth,
vented through volcanoes and cracks in its surface. The evidence indicates that this
was a mixture of carbon dioxide (CO
2
), dinitrogen (N
2
), and water vapor (H
2
O
v
),
with trace amounts of H
2
[
2
]. There was roughly 100 times as much gas in that second
atmosphere as there is in the atmosphere today. Molecules such as CO
2
and H
2
O
v
stored much more heat than the atmosphere does today, creating a greenhouse effect.
The Sun had not reached its full intensity at that time, thus the high concentrations of
these gases helped to prevent the young Earth from freezing.
Most of the water vapor that outgassed from the Earth's interior condensed out
of the atmosphere to form the oceans. The rain absorbed and carried down lots of
CO
2
that was absorbed in the oceans. As life evolved most likely in the primitive
oceans, much of the dissolved CO
2
was transformed into carbonates (CO
2
3
) that
were trapped in the shells of many organisms [
1
]. It is estimated that for each
molecule of CO
2
presently in the atmosphere, there are about 10
5
CO
2
molecules
incorporated as carbonates in sedimentary rocks [
2
]. What remained in the air
were mostly N
2
and H
2
O
v
[
1
]. Since N
2
is chemically inert, sparcely soluble
in water, and noncondensable, most of the outgassed N
2
accumulated in the
atmosphere over geologic time to become the atmosphere's most abundant
constituent [
2
]. Some of the oxygen that entered into the early Earth atmosphere
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